Filter materials of woven cloth, mesh, or felts for use in industrial liquid filtration equipment such as filter presses, rotary drum filters, traveling-belt filters, etc., are well known in the art. The filter materials described above are often used with a pre-coat of diatomaceous earth, perlite, cellulose fibers, gypsum, and the like, in order to accomplish optimum filtrate clarification, reduce the risk of blinding of the filter cloth, and to provide a renewable filter medium which permits longer periods of operation.
Important properties desired of filter materials are particle retention on the surface (vs. in depth), high flow rates, good filter-cake release, resistance to blinding, ease of cleaning (vs. the need for disposal of precoat media), maximum filtrate clarity, and minimal filter-cake contamination. Also important are strength, durability, and dimensional stability for the equipment and process conditions, and chemical stability for the process.
Whether or not such filter materials are used with or without a precoat, difficulties can be encountered in satisfying the often antithetical properties desired of them. For example, when used without a precoat, to obtain clear filtrate from a feed containing fine particles the filter cloth or felt must be quite tight and dense at the expense of filtration rate, cake-release properties, and blinding resistance. Furthermore, cleaning or removal of particles trapped in the filter material is time consuming, expensive, and frequently ineffective. When a filter aid precoat is used filtration rate, filtrate clarity, resistance to blinding, length of operation, and filter-cake removal can be significantly improved, however, other problems can be encountered. The precoating of a filter aid on the filter material is an extra step, requiring additional materials, methods, and equipment. Also, the filtered particles can become mixed with or contaminated by the filter aid which can lead to difficulty and expense in separating and recovering them, or may require an additional waste disposal operation.
To overcome some of these problems, surface filtration materials and techniques have been developed. In some cases surface filtration materials are the same materials described above which have been modified to alter their surface porosity, for example, by flattening the surface with heat treatments or pressing. More often, surface filtration materials include a microporous filtration membrane laminated to a support or backing material. The surface filtration membranes provide high filtration rates, good filtrate clarity, good filter-cake release, and resistance to blinding. They are, however, relatively fragile and can be physically damaged by methods used for cleaning and cake removal such as scraping, liquid sluices or rinses, fluid flow back-pulsing or other flow interruption methods, and the like. As a result surface filtration materials often have shorter service lives, or, have their effectiveness limited by operational compromises required to extend their service life.
Surface filtration membranes may be selected from a variety of porous plastic materials including polyolefins, polyurethanes, polyesters, polyamides, fluoropolymers, and other synthetic polymer materials. The membranes are laminated to support materials to help them withstand the rigors in use and handling associated with the equipment and processes in which they will be used. Support materials are typically textile felts or woven fabrics which are laminated to the membranes by methods well known in art, such as with adhesives, direct heat-bonding of the layers, etc.. The felts and fabrics used for support may be made of the same organic polymers listed above, or of inorganic materials such as fiberglass, ceramic fibers, and the like.